ADS7864YB Burr-Brown Corporation, ADS7864YB Datasheet - Page 15

• Manufacturer Part Number: ADS7864YB
• Description: 500kHz/ 12-Bit/ 6-Channel Simultaneous Sampling ANALOG-TO-DIGITAL CONVERTER
• Manufacturer: Burr-Brown Corporation

LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS7864 circuitry. This is particu-
larly true if the CLOCK input is approaching the maximum
throughput rate.
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connections
and digital inputs that occur just prior to latching the output
of the analog comparator. Thus, driving any single conver-
sion for an n-bit SAR converter, there are n “windows” in
which large external transient voltages can affect the conver-
sion result. Such glitches might originate from switching
power supplies, nearby digital logic or high power devices.
The degree of error in the digital output depends on the
reference voltage, layout, and the exact timing of the exter-
nal event. Their error can change if the external event
changes in time with respect to the CLOCK input.
With this in mind, power to the ADS7864 should be clean
and well bypassed. A 0.1 F ceramic bypass capacitor should
be placed as close to the device as possible. In addition, a
1 F to 10 F capacitor is recommended. If needed, an even
larger capacitor and a 5 or 10 series resistor may be used
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to low-pass filter a noisy supply. On average, the ADS7864
draws very little current from an external reference as the
reference voltage is internally buffered. If the reference
voltage is external and originates from an op amp, make sure
that it can drive the bypass capacitor or capacitors without
oscillation. A bypass capacitor must not be used when using
the internal reference (tie pin 33 directly to pin 34).
The AGND and DGND pins should be connected to a clean
ground point. In all cases, this should be the ‘analog’
ground. Avoid connections which are too close to the ground-
ing point of a microcontroller or digital signal processor. If
required, run a ground trace directly from the converter to
the power supply entry point. The ideal layout will include
an analog ground plane dedicated to the converter and
associated analog circuitry.
APPLICATIONS
An applications section will be added featuring the ADS7864
interfacing to popular DSP processors. The updated data
sheet will be available in the near future on the Burr-Brown
web site (www.burr-brown.com).
ADS7864
®